Linear motors (LM) are used with machine tools since many years, in particular with milling machines, where the high acceleration and high speed achieved, and the fact that no maintenance is needed, are a great plus for the process compared with conventional systems in which rotatory motion is converted in a linear motion by means of a ball screw. LM's have also been proposed for use with Electric Discharge Machines (EDM) since more than 30 years (cp. JP63002626A). Electric discharge machines by Sodick are equipped with linear motor drive systems since 1999, only a few years after the introduction with milling machines.
A major issue of flat linear motors and one of the reasons why they have not replaced conventional systems is the strong magnetic attraction force between the iron core of primary part and permanent magnets (secondary), which has to be absorbed by adequate design of the machine structure. This force can be neutralized by a symmetric constitution of the motor itself, as with the U-channel type linear motor, or by symmetric collocation of motors within the machine.
More recently another well-known type of linear motor has been introduced as actuator for machine tools: the linear shaft motor (hereinafter referred to as LSM). Compared with other types of linear motors, LSM have a number of advantages which make them a very attractive solution for the axis actuation in certain machines. In particular the symmetric constitution leads intrinsically to balanced forces. The structural components of the machine are thus loaded essentially in the desired axial direction (thrust direction), like traditional ball screw transmissions, and structural components do not need to be as stiff as for flat LM's.
The shaft consists mainly in a row of permanent magnets producing a strong magnetic field. With the cylindrical constitution of the shaft, the entire cylindrical surface area of magnets can be used to produce thrust. A possible design of the shaft is shown in U.S. Pat. No. 6,040,642 assigned to GMC Co. Ltd. Ironless LSM provide very smooth velocity and position control, and are very energy efficient.
The LSM is preassembled, so that the mounting on the machine is comparably easy. Also, the compact symmetric constitution of the linear shaft motor is beneficial to its integration with machine tools; the motor is mounted in close relation to the respective machine component achieving excellent stiffness in force transmission. However, since the LSM is such directly linked to the respective machine component, at the core of the machine structure, the heat produced in the motor forcer must be evacuated as good as possible to avoid elongation and deformation of the machine structure and consequently a loss of accuracy.
The heating power due to ohmic losses in the copper windings is proportional to I2×R. In normal operation, the heat generated continuously by each LSM in the range considered herein is in the order of several hundred watts. It is to be noted that normal operation is a quasi-permanent condition. Generally a CNC machine tool comprises at least three axis motors, whereas a wire electric discharge machine comprises typically two horizontal axis pairs, and a fifth vertical axis which employs rotary actuation. It goes without saying that the heat immission at the core of the machine structure has to be avoided or at least limited. With the ever more challenging accuracy requirements of machine tools, the heat immission must be intercepted at the source.
Patent document WO2010049969 assigned to Mitsubishi refers to a linear shaft motor equipped with a liquid cooling system comprising a cooling plate (motor coil holder 14) between the motor case and the machine body and two lateral cooling plates. However this cooling concept does not effectively protect the machine body, since a heat transfer to the attachment surface is not avoided. Also the insertion of such plates around and especially above the motor body increases the size of the slider, and the required volume within the machine tool.
In 2012 company Mitsubishi has launched a new series of WEDM (MV-series) using linear shaft motors. Here the problem of heat evacuation has been solved essentially by collocating some cooling plates around the LSM. Of course the height of the LSM with the cooling plates increases significantly.
Patent document U.S. Pat. No. 6,323,567 assigned to Nikon relates to a circulating system for cooling the coil array of shaft-type linear motor, by which the outside surface of the motor is maintained at a set temperature during operation. The coil housing is used to support the coil assembly and provide a fluid passageway to cool the coil assembly. The fluid passageways are distributed around the coils. The coil housing comprises an end section at each end, with fluid inlets/outlets on top of these end sections. The overall size of the shaft-type linear motor by Nikon increases substantially to fit the cooling circuit in the coil housing.
Patent document WO2016025975 assigned to Anca refers to a linear motor, including a mover having a cylindrical body with the coils inside said cylindrical body. The slider has a flange at one end by which the linear motor is securely and easily fixed in the respective machine component. A substantially coolant space is formed between the cylindrical body and the mover housing. Such design requires a considerable space between the moving parts.
Above mentioned cooling circuits are effective, however there is an important limitation, that the cross section of the motor is increased substantially in all directions. In fact the cooling circuits most generally consist in a jacket which is put around the heat source. The size of the motor increases, which in certain cases represents an unacceptable machine design limitation.
The utility model TWM505992U assigned to Excetek displayed in FIG. 1 discloses a WEDM using linear shaft motors. Here the main topic is the implementation of the good rules of mechanical engineering. Here the slider of the LSM is fixedly mounted on a structural component of the machine via its top surface. One thing that is visible here is, that the height of the slider is relevant for its integration between machine parts.
Thus, there is a need for an optimized LSM for high accuracy applications, having smaller size, and particularly a smaller height but an excellent performance and efficient cooling. To take full advantage of the strengths of a LSM in view of best accuracy of the machine tool in which the LSM is used, it is necessary to provide an integrally optimized design.